In most magnetic recording materials presently used, thermoplastic resins, such as vinyl chloride-vinyl acetate copolymer resins, vinyl chloride-vinylidene chloride copolymer resins, cellulose resins, acetal resins, urethane resins, acrylonitrile-butadiene resins and the like are used as binders independently or in combination of two or more thereof. However, these materials suffer from the defect that when used for a magnetic tape they are removed and contaminate the running passage of the magnetic tape such as guide posts, magnetic heads, cylinders, etc., due to inferior abrasion resistances of their magnetic layers.
In addition to the above materials, thermosetting resins such as melamine resins, urea resins and the like, or cross-linkable binders obtained by adding cross-linking agents such as isocyanate compounds, epoxy compounds and the like have also been used as binders for magnetic tapes. However, use of cross-linkable binders cause two serious disadvantages that (1) a resinous solution in which a magnetic substance has been dispersed has a poor storage stability, that is, a short pot life and, therefore the homogeneity of the physical properties of the magnetic coating composition cannot be maintained which, in turn, deteriorates the uniformity of the magnetic tape, and (2) after coating and drying the magnetic coating composition, the coated composition must be subjected to heat treatment for hardening the coated layer whereby the process requires a prolonged time to obtain the final magnetic recording materials.
With the intention of obviating the above-described disadvantages, methods of producing magnetic materials in which mixtures of the olygomers and the monomers of acrylic acid ester type are employed as binders and their magnetic layers are hardened by irradiation with electron beams subsequent to the drying step are disclosed in Japanese Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 15104/72 (French Patent No. 2,120,184 and German Patent No. 2,100,037), 77433/75 and 25235/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and U.S. Pat. Nos. 3,104,983 and 4,004,997. However, such methods cannot provide magnetic recording materials having both high electromagnetic characteristics and excellent abrasion resistance.
In recent years, a demand for magnetic recording materials having high electric characteristics has increased. For this purpose, the magnetic layers must have smooth surfaces to minimize drop out, that is to say, it causes a drop in output, and contributes to the generation of noise. For this reason, magnetic recording materials having smooth magnetic layer surfaces have been required. To this end, the magnetic recording materials are generally subjected to a smoothening treatment after the drying step of the coated layer. However, the methods disclosed in the above Japanese Patent Publication No. 12423/72, and Japanese Patent Application (OPI) Nos. 15104/72 and 77433/75, in which the smoothening treatment is carried out after the hardening treatment by means of irradiation with electron beams, cannot provide sufficient smoothness to the surface. Therefore, these methods cause great noise so far as electromagnetic characteristics are concerned. In addition, the method disclosed in Japanese Patent Publication No. 25235/81 which comprises effecting a preliminary irradiation after coating and drying a magnetic layer, a smoothening treatment and a hardening treatment by irradiation with electron beams, suffers from the disadvantage that a proper dose of the preliminary irradiation must be selected depending upon the kind of binder used and control of the optimum conditions for the preliminary hardening treatment is not easy. Further, the above conventional methods are also undesirable because they do not provide a satisfactory abrasion resistance to a magnetic layer. One reason for this inferior abrasion resistance is presumed to be microscopic destruction caused in the hardened binder of the magnetic layer by the smoothening treatment.
As a result of extensive studies for improving these conventional methods, i.e., the method of using a thermoplastic resin or a thermosetting resin, the method of adding a binder which undergoes cross-linking through a certain chemical reaction, the method of using a binder which can be hardened by cross-linking by irradiation with electron beams, etc., the present inventors completed the present invention.